Unable to send files over Bluetooth OPP

DevicPickerActivity switches to an androidx fragment in layout of bluetooth_device_picker.
But android.app.Activity doesn't support androidx fragment.
So, let DevicePiackerActivity extend androidx FragmentActivity.

Change-Id: I5da98a5e2242b75d2205a786bf373b81dfe6f613
Fixes: 112010509
Test: manual test, robo test

src/com/android/settings/bluetooth/DevicePickerActivity.java

@@ -16,16 +16,17 @@
 
 package com.android.settings.bluetooth;
 
-import android.app.Activity;
 import android.os.Bundle;
 
 import com.android.settings.R;
 
+import androidx.fragment.app.FragmentActivity;
+
 /**
  * Activity for Bluetooth device picker dialog. The device picker logic
  * is implemented in the {@link BluetoothPairingDetail} fragment.
  */
-public final class DevicePickerActivity extends Activity {
+public final class DevicePickerActivity extends FragmentActivity {
 
     @Override
     protected void onCreate(Bundle savedInstanceState) {